Digital tone detection and generation is useful in telecommunications, such as in dual-tone multi-frequency (DTMF) used in "two tone" or "multi-tone" signaling. Known in the art are Sinerom, Taylor series and discrete fourier transform methods of generation. These methods are computationally complex and require a large storage memory. For tone detection, analog filtering of tones followed by analog energy detection is employed.
Conventional tone detection apparatus typically employs an analog tone filter which receives an input signal. The tone filter is typically a high-"Q" (quality-factor) providing extremely sharp rise and fall characteristics around the resonant frequency. Such analog circuitry is complex and costly to manufacture. More importantly, its response is sensitive to changes in temperature, voltage, component values and the like which result in variations in gain and band-pass characteristics.
The tone detection apparatus employs an energy detector following the tone filter responsive to the filtered signal which requires that a "confidence level" be selected. The energy detector produces a signal which is monitored by a decision device which determines whether the detected energy exceeds a "threshold" level and produces a binary-valued output therefrom indicating whether the received signal has the appropriate frequency at a satisfactory energy level.
The time required to detect a tone by such apparatus is determined by the Q factor and confidence level selected. This time can be substantial if a high level of quality and confidence is required.